<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>下拉框多选</title>
</head>

<body>
    <select multiple="multiple" size="3" id="sel">
        <option value="volvo">选项1</option>
        <option value="saab">选项2</option>
        <option value="mercedes">选项3</option>
        <option value="audi">选项4</option>
    </select>
    <br>
    <button id="btn">查看选中选项</button>
    <script type="text/javascript" src="jquery.js"></script>
    <script type="text/javascript">
        $(function(){
            $("#btn").on("click",function(){
                console.log($('#sel').val())
            })
        })
    </script>
</body>

</html>